To increase operating efficiencies of modern aircraft engines, it is desirable to decrease weights of airfoiled component parts such as jet engine fan blades, exit guide vanes, aircraft propeller blades and certain structural support members positioned in air streams. Substantial decreases in weights of such components have been achieved through use of composite materials including for example graphite fiber reinforcements with an epoxy matrix. Composite components, at their leading edges, fail to provide adequate strength to protect themselves from erosion and foreign object damage and especially from damage as a result of leading edge impact with birds, ice, stones, sand, rain and other debris. Adding further to their fragility, these components are usually quite thin and as a result even more susceptible to foreign object damage. Accordingly, protective sheaths are often used to protect the leading edge.
The art of manufacturing electroformed sheaths is well known, as described for example in U.S. Pat. No. 4,950,375 to Leger. Typically a die or mandrel, made of conductive material such as titanium, is formed to have an exterior surface that conforms to a blade's airfoil configuration minus the thickness of the sheath to be electroformed on the mandrel. Desired thicknesses of the sheath are achieved by a well-known process of shielding, wherein barrier walls or shields are placed adjacent the mandrel in such positions that shields direct the flow of electrical current between the anodes and the exposed work surface of the mandrel in the electroplate solution. For example, where a sheath leading edge must be thicker and hence stronger than a sheath trailing edge, the shield portion adjacent a first surface section of the mandrel palm, which has the form of the sheath leading edge, would be positioned a greater distance from the surface section of the mandrel than a shield portion adjacent a second surface section of the mandrel palm, which is forming the sheath trailing edge. After the mandrel has been in the electroplate bath for a predetermined time, it is removed. The electroformed sheath is next mechanically removed from the mandrel. The sheath exterior may then be machined to provide a smooth, aerodynamic contour. It is then pressed fit over and bonded to the composite blade, in a manner well-known in the art.
A material typically used in the quest for achieving leading edge protection is a nickel electroform produced from modified Watt's nickel electrolyte. This material is very hard, greater than 600 VHN, and is inclined to be relatively brittle. It may be subject, therefore, to cracking, fracture, chipping, and breaking apart upon a direct 90.degree. impact by foreign objects. Another material for use in erosion resistance is titanium 6Al-4V, which is typically used for the construction of most jet engine fan blades. Still another material which is used in forming electroformed sheath is hardened sulfamate nickel.
Table 1 is indicative of an example of the erosion and impact resistance for modified Watt's nickel (700 VHN), hardened sulfamate nickel (470 VHN) and titanium 6Al-4V, at 20.degree. and 90.degree. angles, with sand as the erosion medium traveling at 800 ft/sec. The hardened sulfamate nickel results were used as the baseline measurement for the remaining erosion determinations. As can be seen, the titanium is clearly surpassed by the Modified Watt's nickel and hardened sulfamate nickel in erosion rate and total erosion at both the 20.degree. and 90.degree. angles. Not only did the titanium perform relatively badly in these tests, it is also very expensive and typically only used with high output jet engines which have financial budgets and structural requirements far exceeding those of other aerospace components, such as propellers. Due to the expense of the titanium, it would be desirable to have a more economically and better performing electroform sheath for protection of jet engine components, so as to reduce the cost of the same. With particular respect to propeller blades, recent events have indicated an alarmingly fast rate of replacement, due to breakage, of modified Watt's nickel electroformed sheaths, which are highly used in the industry. The high replacement rate seems due to a combination of the hardness, brittleness, and low ductility, of this material, as indicated above. In testing, and at a 90.degree. impact angle, the Watt's nickel cracked and chipped. In certain areas of the world, the modified Watt's nickel electroformed sheaths have eroded quite substantially at the tip of the propeller blade, requiring replacement of the sheath within as little as 70 hours of operations, wherein a typical mean time between unscheduled returns for such maintenance is 1,000 hours, and preferably 5000 hours.
There exists a need, therefore, for an electroformed sheath formed from a material or composition which is relatively inexpensive and exhibits improved erosion resistance at a variety of impact angles and other conditions, in comparison to currently used and known materials.